Few pieces of interesting information, stolen from Reddit:
At level 27, you are just over halfway to level 30. (1,100,000/2,000,000)
At level 30, you are 10% of the way to level 40. (2,000,000/20,000,000)
At level 37, you are less than halfway to level 40 in total xp. (9,500,000/20,000,000)
At level 37, power wise, you're 95.7% of the way to lvl 40, though, because powering up Pokemon past lvl 30, the CP increases per level are diminished.
At lvl 30 you're at 85.8% of the power potential of your Pokemon.
Consider level 30 the "Softcap" for the game since at level 30 you can already catch the top level wild Pokemon, and the gains on your Pokemon are minimal, the costs to increase their CP skyrocket while their CP gains are minimalized.
To get a lvl 30.5 Blastoise to 40.5 it costs 160,000 Stardust and 205 candy for a gain of 363 CP
To get a lvl 1 Blastoise to 30.5 it costs 129,800 Stardust and 128 Candy for a gain of 2162 CP